Eosin is a fluorescent acidic / negative compound that binds to and forms salts with basic, or eosinophilic, compounds containing positive charges (such as proteins that are basic / positive due to the presence of amino acid residues such as arginine and lysine) and stains them dark red or pink as a result of the actions of bromine on fluorescein. In addition to staining proteins in the cytoplasm, it can be used to stain collagen and muscle fibers for examination under the microscope. Structures that stain readily with eosin are termed eosinophilic.